Lyme disease is a major public health problem worldwide and is the most common arthropodborne infection in the United States. Borrelia burgdorferi, which is the cause of Lyme disease in North America, is an extracellular pathogen that has the unique ability to evade the host immune response and infect individuals for years to decades. To gain a better understanding of how this organism persists in the infected mammal, many laboratories, including our own, have focused on identifying and characterizing outer surface proteins of B. burgdorferi. Since the interface between B. burgdorferi and its mammalian host is its outer surface, proteins localized in the outer membrane must play an important role in dissemination, virulence, tissue tropism, and, ultimately, immune evasion. Over the last funding period, we have identified and focused on several outer surface lipoproteins (Osps) from B. burgdorferi. We have determined the role of numerous surface lipoproteins in pathogenesis and examined their potential as vaccine candidates. While we continue to be very productive in characterizing surface lipoproteins that are capable of binding factor H to help aid the spirochete in evading serum-mediated killing during the acute stage of infection, we have yet to identify a specific lipoprotein that could work as a potential vaccine candidate for Lyme disease. The lack of good vaccine efficacy for the various lipoproteins we have been characterizing over the last funding period prompted us to also start examining integral, membrane spanning, outer membrane proteins (OMPs). These more recent studies led us to discover the protein machinery required for chaperoning and assembling OMPs into the B. burgdorferi outer membrane. Although the mechanism of outer membrane biogenesis is poorly understood in spirochetes, our recent studies have indicated that a heterooligomeric OM protein &#946;-barrel assembly machine (BAM) complex is required for proper assembly of OMPs into the borrelial OM. The B. burgdorferi BAM complex is made up of the integral outer membrane protein BamA, and two additional periplasmic lipoproteins located in the inner leaflet of the outer membrane, BB0324 and BB0028. The borrelial BAM complex is not only required for inserting OMPs into the borrelial outer membrane, but we also recently published that the BAM complex plays an important role in exporting lipoproteins to the surface of B. burgdorferi. The studies we propose in this renewal application will allow us to characterize the BAM complex and examine in detail its role in export and translocation of both OMPs and Osps to the surface of this spirochetal pathogen. A better understanding of the basic mechanisms by which the BAM complex chaperones and exports OMPs and Osps to the surface of B. burgdorferi will also allow us, for the first time, to begin elucidating the complete outer surface proteome of B. burgdorferi. Additionally, since the BAM system is a conserved pathway for OMP export and localization among all Gram-negative bacteria, the studies outlined here also could lead to the development of novel antibacterial compounds that could have an impact not only against spirochetes, but could potentially be widely applicable against other important human pathogens, such as E. coli, Salmonella, Vibrio, Pseudomonas, Neisseria, Helicobacter and others.